Timing belt (and water pump) .. if it's not done now, do it. While I've seen them go 130k, I would do them every 60-80k miles. If it does slip or break, you'll experience the pain of replacing a lot of intake valves on top of pulling the engine and turbos, as that's the only way to get the heads off. If no one can tell you if it's been done or not, peer at the belt by prying back the timing belt cover a few mm and look at the belt with a flashlight. If it looks fresh and new and black, it's new. If it looks worn and old, well.. it's not.

Coolant: Right now it should be fresh and bright pink, not brownish or orangish. Don't base it off the coolant overflow bottle, as it will be somewhat discolored by now. But, those are only $28 at the dealer and I recommend replacing them as they come with a new cap, if/when you do the coolant flush. It's long life coolant, but 100k miles is a long time and that's when it should be flushed. The only drain is the block drain, located between the trans and engine directly underneath looking straight up through the bell housing. It's an 8mm green hex key bolt, it will be very tight and need an 8mm hex driver and breaker bar, and needs a new O-ring (available at the dealer) once removed. You need a vacuum coolant filler to put coolant into these engines properly, so either borrow one or pay the dealer to do this job for you.

Coolant leaks: Inspect for any pink crustys. Particularly look into the V of the engine from above once the middle engine dress cover is removed, peering in at a 45 degree angle through the small gap below the throttle body. If you see any pink in there, your after run pump or coolant hardline is leaking. Also look in back of the engine on the passenger side. Any pink crustys = leak.

Battery : If it looks old and original, the cells probably need topping off with distilled water. It's a maintenance item but few actually know that it's supposed to be topped off regularly. Just twist each cell cover off and there's a little MIN/MAX bar on each one.

Diverter valves: While looking for coolant leaks, start the engine up and reach normal operating temperature. Put your hand on the two black roundish looking valves, roughly 3" in diameter that point towards the throttle body. There will be small vacuum lines running to each. If you feel either "vibrating" or making fluttering or honking noises at idle, it might be shot. To look further, if you pinch off one of the vacuum lines with a pair of pliers and it goes away, that valve is shot. They die often, aftermarkets
that don't break (they use a piston instead of a diaphram) are available for a couple hundred bucks. Highly recommended as I ate through about 4 sets of the updated TT ones by 96k miles. They decrease strain on the turbos when you lift off the throttle, so.. when they're dead, well... there's more strain = wear and tear.

Oil leaks: Check the rear passenger and front drivers side of the V of the engine. Any wetness would be the legendary timing chain tensioner gaskets. They'll almost never leak so bad as to create a steady drip of oil, but they will start to leak and get wetter and wetter, making some mess as mileage goes up. These can be spendy to have replaced, with dealers charging around 8 hours of labor for both sides. If you've not done it before, I wouldn't try it yourself as you can drop tiny bits into the engine. I used to use a small magnet to catch them. Valve covers- these start getting wet at 100k miles. Loosening all the 10mm nuts that hold them on and then re-torquing to 115 INCH/lbs working from the center nut outwards in a clockwise pattern, slowly but surely, they may stop leaking forever. Or you can just change them.

Spark plugs: Even though they're double platinum the engine still eats them like candy. Misfire codes are a telltail sign. I'd swap them every 25k miles if you like to get on it.

Oil: 5W-40 synthetic is a good idea. 0W-40 German Castrol is nice too, 0W-30 works alright. Basically, anything synthetic is great for this engine, and non synthetic is bad. You can tell what it's had all it's life by looking at the oil fill cap. Take it off and look inside. If it's shiny metal, it's had synthetic all it's life. If it's crusty and caked and brown, it's not. The more gelatinous cake under the cap, the less synthetic oil it's seen in it's life.

Interior: The dot matrix display is a common failure. New gauge clusters have bugfixed designs that don't drop dots or lines (usually) but fetch a high price, $750 or so (remanufactured, which is good cuz it's bugfixed). I'm a big fan of Stabilant 22 and CAIG's DeOxit D5, but I've not read of a success using it on this problem. It does resemble a connection type of problem between the display and the driver board, as mine used to drop a line but only when it was really cold out. The gauge cluster comes out without touching the rest of the dashboard, it's held in with a couple of torx screws accessible by popping the top steering wheel trim off. You'll have to reach in behind once the cluster is out a few inches and pop the
spring-cam-lock connectors (all three) on the back kind of blind to get it all the way out.

Speakers: If they rattle with bass, they're probably in need of replacement. The Bose Symphony audi system doesn't take kindly to aftermarket replacements (it ends up sounding like crap) and the factory replacements are around $90 each. In a sedan, the rear speakers are accessed from above, not below. Kind of a pain. The door panels are much easier to remove. Stereo: If it changes channels on you, it's possessed. They do that sometimes. If it stops working, it'll need replacement. They've gotten MUCH
cheaper last I checked. < $200 at the dealer now for a reman if you provide your old one as the core.

Rear diff: Sometimes the seals on the output shafts get a little wet.

Wheel bearings: They'll be either freshly replaced, or needing it soon. They like to abandon ship at 100k miles. But they do so rather gracefully, giving you 10-15k miles of warning with a steady increase in rattling noise.

O2 sensors: These don't like to live forever. There are 4. The rear 2 are easy. The front 2 ... well. Only easy if you've done them before.

Coolant temp sensor: These get flakey. They're easy, check out AudiWorld for instructions. If the temp display is anything but in the middle at normal operating temps, the sensor is toast.

Suspension arms: If it clunks over bumps, it'll need them. I recommend the VW Passat suspension arm kit from the dealer, it's 4 arms for the price of 1 Audi one, and it's the identical part in every way shape and form, including the part #. These can be installed without an alignment of any sort.

If well taken care of, 5k synthetic oil changes religiously, allowed to warm up (one notch up on the oil temp gauge) before you get into the boost [which can take an agonizingly long time], and cooled down (go slow the last couple of miles) when hot, the turbos will last 200k miles. If oil changes get skipped, you like to get into the boost while backing out of your driveway on wintery mornings, and you drive around in 100 degree heat like a madman and arrive at your destination and flick the key off and walk away immediately, the turbos won't make it to that point.

There are failures unfortunately, and when they do fail it's talked about very vocally because the price is extreme: $4-5k to have them replaced. They must be done by the pair. If you chip it, it's going to add another factor to this equation (heat) and make it more likely to fail (but not necessarily so with much care and diligence).

I liked to clean my climate control buttons if they got sticky with 70% isopropyl alcohol. I'd just spray it on and wipe it off with a cotton cloth a minute later, padding gently to soak it up. Repeat until the button frees
itself. Someone else mentioned just water, which works too but sometimes takes a bit too long to dry and also might not attack the dried out Coca-Cola sufficiently well.

That about sums it up for my Sunday morning. Hehehe. They're great cars, I miss mine still and I've got an '05 S4 Avant. I had mine for 96k miles before FOOLISHLY selling it in pristine condition.

Oh and just for reference: The car should come with two transmitter key fobs, a valet key (it won't unlock the trunk), and a plastic thin wallet key. (4 keys total). The radio manual should have the security code sticker in it. Don't let the dealer try to tell you that they can't do anything about those being missing- they can cut/make/reprogram them all with blanks on hand and I wouldn't sign for the car unless they hand you all four.

Stereo: If it changes channels on you, it's posessed. They do that
sometimes. If it stops working, it'll need replacement. They've gotten MUCH
cheaper last I checked. < $200 at the dealer now for a reman if you provide
your old one as the core.

Rear diff: Sometimes the seals on the output shafts get a little wet.

Wheel bearings: They'll be either freshly replaced, or needing it soon. They
like to abandon ship at 100k miles. But they do so rather gracefully, giving
you 10-15k miles of warning with a steady increase in rattling noise.

O2 sensors: These don't like to live forever. There are 4. The rear 2 are
easy. The front 2 ... well. Only easy if you've done them before.

Coolant temp sensor: These get flakey. They're easy, check out audiworld for
instructions. If the temp display is anything but in the middle at normal
operating temps, the sensor is toast.

Suspension arms: If it clunks over bumps, it'll need them. I recommend the
VW Passat suspension arm kit from the dealer, it's 4 arms for the price of 1
audi one, and it's the identical part in every way shape and form, including
the part #. These can be installed without an alignment of any sort.

If well taken care of, 5k synthetic oil changes religiously, allowed to warm
up (one notch up on the oil temp gauge) before you get into the boost [which
can take an agonizingly long time], and cooled down (go slow the last couple
of miles) when hot, the turbos will last 200k miles. If oil changes get
skipped, you like to get into the boost while backing out of your driveway
on wintery mornings, and you drive around in 100 degree heat like a madman
and arrive at your destination and flick the key off and walk away
immediately, the turbos won't make it to that point.

There are failures unfortunately, and when they do fail it's talked about
very vocally because the price is extreme: $4-5k to have them replaced. They
must be done by the pair. If you chip it, it's going to add another factor
to this equation (heat) and make it more likely to fail (but not necessarily
so with much care and dilligence).

I liked to clean my climate control buttons if they got sticky with 70%
isopropyl alcohol. I'd just spray it on and wipe it off with a cotton cloth
a minute later, padding gently to soak it up. Repeat until the button frees
itself. Someone else mentioned just water, which works too but sometimes
takes a bit too long to dry and also might not attack the dried out
Coca-Cola sufficiently well.

That about sums it up for my sunday morning. Hehehe. They're great cars, I
miss mine still and I've got an '05 S4 Avant. I had mine for 96k miles
before FOOLISHLY selling it in prestine condition.

Oh and just for reference: The car should come with two transmitter key
fobs, a valet key (it won't unlock the trunk), and a plastic thin wallet
key. (4 keys total). The radio manual should have the security code sticker
in it. Don't let the dealer try to tell you that they can't do anything
about those being missing- they can cut/make/reprogram them all with blanks
on hand and I wouldn't sign for the car unless they hand you all four.

Remember: Be picky. It's an Audi. The engineers were picky, there's no
reason why the customers can't be. :)